Van Nostrand's Scientific Encyclopedia defines a transformer as being a device for transferring electrical energy from one circuit to another by magnetic induction, usually with an accompanying change of voltage. There are no moving parts; nor is there any electrical connection between the two circuits (except in the case of an auto-transformer). The electrical energy is transferred via the magnetic linkage. The energy-supplying circuit is termed the “primary”, and the energy-receiving circuit is termed the “secondary”.
Some transformers employ three-dimensional bobbin-wound coils. Others employ relatively-planar or substantially two-dimensional sheet windings that are embedded in various dielectric layers of a printed circuit board. For example, U.S. Pat. No. 6,353,379 B1 discloses a transformer spanning multiple printed circuit boards. Each board has a multi-layer structure. Sheet windings are embedded in the various layers. A magnetic core, such as an E-core, may be mounted on the main board. The ostensible purpose of the device is to lower the profile of the dc-dc converter.
U.S. Pat. No. 5,990,776 discloses another type of transformer having sheet windings embedded in various layers of a multi-layer printed circuit board.
U.S. Pat. No. 4,547,961 discloses a miniaturized transformer having sheet windings embedded in various layers of a multi-layer printed circuit board. An E-core embraces the windings.
These references demonstrate that there is an existing and ongoing need to provide a low-profile transformer that utilizes printed circuit board technology, that uses standardizable off-the-shelf components to minimize the number of different component parts in the assembled transformer, and that may use add-on piggyback boards to selectively vary the properties of the transformer.